The enzyme myosin light chain kinase contains three domains that interact with one another. These are an active site, responsible for the transfer of phosphate from ATP to myosin; a calmodulin-binding site responsible for the activation of the enzyme by Ca-calmodulin; and two sites that can be phosphorylated by cAMP-dependent protein kinase. We are studying the mechanism by which these sites interact with one-another, using the techniques of brief proteolysis to generate relevant peptides.